Lead-acid storage batteries are widely used in commercial, industrial and military applications. The largest single application of lead-acid storage batteries is for the starting, lighting and ignition (SLI) of automobiles, trucks and buses. These batteries are charged automatically from a generator driven by the engine while it is running, they supply power for the lights while the engine is shutdown and for ignition and cranking when the engine is started. Other applications include SLI for boats and aircraft, the storage of electromotive power for electric vehicles; and the regulation of electric power for industrial generators, a typical non-motive situation.
The conventional lead-acid battery is use today contains a series of lead-acid cells, each including a positive plate containing positive, active, material such as lead dioxide, and a negative plate containing negative, active, material such as sponge lead immersed in an electrolyte solution, typically dilute sulfuric acid. The respective positive and negative plates are connected in parallel, with the power or current output of a cell being determined by the number and size of the plates. The open circuit potential developed between each pair of positive and negative plates is about two volts. Since the plates are connected in parallel, the combined potential for each cell will be also about 2 volts regardless of the number of plates utilized in the cell. One or more cells are then serially connected to provide a battery of desired voltage. Common low voltage batteries are 12 volt batteries having 6 serially connected cells.
The positive and negative plates are usually oriented vertically in a horizontal stacked relationship. As a result of this vertical orientation, electrolyte stratification commonly occurs vertically along the plate surfaces. This results in diminishing battery performance with time. Another problem with conventional lead-acid batteries is their limited lifetime due to shedding of the active materials from the positive and negative plates. Pasted plate lead-acid batteries are by far the most common type of lead-acid battery. Typically, a paste of lead oxide is applied to the surfaces of the positive and negative grids. When a initial electric charge is applied to the plates, the lead oxide paste on the positive grid is oxidized to lead dioxide while the lead oxide on the negative plate is reduced in the cathodic reaction. During continued operation of the lead-acid battery, shedding or flaking of the deposited lead paste occurs. The flakes of material fall down between the vertically oriented plates and accumulate in a well on the battery bottom. After a period of time, sufficient flakes accumulate on the battery bottom to short circuit the negative and positive grids resulting in shortened battery life.
Conventional lead-acid batteries are inherently heavy due to the use of lead in constructing the plates. This is unacceptable in applications where a lightweight battery with a high power density is required, such as for use in aircraft and electric cars. Emphasis in the prior art was placed on producing thinner plates made from lightweight materials used in place of or in combination with lead. Although the thinner, lightweight plates were beneficial in reducing battery weight, they presented structural design problems. Cell structures which were sufficiently strong and rigid to prevent structural failure during normal use were then required.
In my U.S. Pat. No. 4,405,697, entitled "IMPROVED LEAD-ACID BATTERY," the disclosure of which is expressly incorporated herein by reference, a lightweight battery is described which includes a plurality of horizontally oriented, vertically stacked alternating positive and negative monoplates or grids. Tabs are provided extending from two opposite edges of the plates or grids along the total length of the grids on both sides thereof. The negative and positive plates were stacked so that two positive tabs extend from the cell or grid stack on sides adjacent the two negative tabs. The common tabs on each side of the grid stack were welded together in parallel to form four bus bars or plates extending vertically up the cell sides. The bus bars not only greatly reduced the electrical resistance in the battery cell or grid stack, but additionally provided rigidity and strength to the cell structure. Further, the horizontal orientation of the grids prevented the accumulation of flaked lead compounds at the battery bottom, since their downward movement was blocked by the glass mat containing the electrolyte placed between each set of positive and negative plates. Also, stratification of the electrolyte was avoided since the electrolyte was confined and contained within the acid resistant glass mats by capillary action. Oxygen generated within the chamber during discharge was permitted to escape via a single resealable vent. This construction avoided the buckling, warping and unequal gassing which occured in batteries using separate, sealed cells.
In order to increase the available voltage potential of the "IMPROVED LEAD-ACID BATTERY," which is necessary for most applications, it was necessary to serially connect a number of cells together. In my U.S. Pat. No. 4,539,268, entitled "SEALED BIPOLAR MULTI-CELL BATTERY," the disclosure of which is expressly incorporated herein by reference, bipolar plate groupings, secured within the battery stack, were placed between alternating monopolar plates. The positive ends of the bipolar plate groupings were located adjacent the monoplates having negative active material and the negative ends of the bipolar plate groupings were located adjacent monoplates having positive active material. Each bipolar plate grouping included one or more bipolar plates with electrolyte layers between the bipolar plates. The bipolar plates conducted current perpendicular through the plates, resulting in low-resistance serial electrical connections. Bus bars connected to the edges of the monoplates provided structural support and electrical connection with the battery terminals. Varying the number of monopolar and bipolar plates in each grouping afforded a convenient way to customize the voltage potential produced by the battery for the specific application. The number of bipolar plate groupings connected in parallel by the bus bars, and the area of the plate surfaces, determined the discharge current capacity of the battery. Battery manufacturers enjoyed the versatility to easily adapt battery output current and voltage characteristics to a specific application.
For many years, battery manufactures have searched for ways to penetrate new lucrative markets and expand existing markets for their products. One problem for which a solution has been sought is that the state of the art in end-plate cell stack battery designs are still too heavy and bulky for many practical applications. The need to provide pressure and rigidity on the cell stack end-plates to inhibit warping or bowing of individual plates and the need to have very low resistance terminals has made the extensive use of lead preferable, which adds to the weight of the design. Another problem for which a solution has been sought is that the awkward protrusion of battery terminals from the enclosure effectively increases the working volume of the battery and many times prohibits the convenient networking of multiple bipolar storage batteries in parallel or serial arrays. Still another problem is an undesirable tendency for the development of substantial operating temperature differentials between the end cells and the interior cells in the bipolar cell stack. This anisotropic tendency results in non-uniform cell performance and lower power efficiency. Solutions to these problems are necessary for growth of the battery industry.
A high demand for bipolar storage batteries in electric vehicles, portable consumer products, industrial load leveling, and many airborne applications would be extremely desirable to the battery industry. Clearly, the need for lighter weight, lower volume and higher effeciency batteries is essential for growth. For example, though an economical, high performance and environmentally safe electric vehicle has been sought for many years, the current state of the art in bipolar storage batteries has not resulted in a viable consumer product. The next advancement in the state of the art in bipolar storage batteries will stimulate growth in the battery industry through the development of new designs exhibiting improved thermal performance and higher power [and energy] to weight [and volume] efficiencies.